French Jew, an outstanding Talmudic authority of his time, who was responsible for a series of far-reaching decisions governing relationships between Christians and Jews in medieval Europe. He was also one of the most eminent of the French tosaphists (commentators on particular passages in the Talmud).

Tam was the grandson of Rashi, the renowned 11th-century Talmudic commentator. As a symbol of Jewry, he was attacked in 1147 by a band of crusaders, who wounded his head five times as revenge for the five wounds that the Jews allegedly inflicted on Christ. Saved from death by a passing knight, he fled to neighbouring Troyes. There he became a leading participant in the rabbinical synods that began about 1160.

The synods developed rules to govern the relations between Christians and Jews, who were living on closer terms because of changing historical conditions. Tam was foremost in settling the terms from the Jewish side. The key ordinances of Rabbenu (“Our Teacher”) Tam provided that (1) disputes between Jews were to be resolved by the Jewish authorities; (2) the law of Rabbenu Gershom (c. 960—c. 1028/40) abrogating polygamy was essentially reinforced; and (3) no Jew could lightly challenge the legality of a Jewish deed of divorce.

Tam's major legal work is Sefer ha-yashar (first published in 1811 in Vienna; “Book of the Righteous”). It contains explanations of 30 tractates of the Talmud, as well as responsa (authoritative answers to questions about Jewish law). He also wrote religious poetry, some of which was later incorporated into the Hebrew prayer book.